Why does cis-1,2-dimethylcyclobutane achiral?
I am studying organic chemistry now. I think the molecule is chiral because the ring of cyclobutane isn't plane, so there is no symmetry plane can be found in this structure. But the fact is that it is an achiral molecule. Is it because of ring flip? If not, what's the reason? Hope you can explain it in detail. Thanks.
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Google inversion barrier cyclobutane. As you will see the barrier to inversion of the puckered square conformations of cyclobutane is around 6 kJ mol^-1. This is an extremely low barrier and once the ring passes through the square intermediate the molecule has a C2 and mirrors and the Me groups are equivalent: nonchiral from group theory. The lowest barrier you can detect different environments by NMR is about 25 kJ mol^-1 and then at -100°C! To separate two chiral isomers you need a barrier >~ 100 kJ mol^-1 (rates <~10^-2 s^-1). As you know :NRR'R" cannot be resolved into enantiomers due to rapid inversion (but :PRR'R" can!).
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